Among methods for inspecting metallic materials is the eddy current flaw detection. The eddy current flaw detection by means of the insertion type eddy current flaw detection probe is widely employed in the inspection of non-magnetic tubes made of such metals as austenitic stainless steel, titanium or copper alloy.
When inspecting a magnetic tube made of carbon steel, ferritic stainless steel, duplex phase stainless steel (or tow-phase stainless steel) consisting of ferrite phase and austenite phase or the like, flaws cannot be detected accurately since the eddy current flaw detection probe for non-magnetic tubes causes eddy currents to flow only in the surface and sensitivity of the detector is adversely affected by a noise attributable to local variation in magnetic permeability.
A known eddy current flaw detection probe for duplex phase stainless steel heat transfer tube has such a constitution as detection coils are disposed around a central portion of a cylindrical yoke in the direction of the cylindrical axis thereof, and permanent magnets are disposed around the yoke on both sides of the detection coil in the direction of the cylindrical axis so that the direction of magnetization lies in the radial direction of the yoke and the magnetic poles on the yoke side are different from each other (refer to, for example, Non-Patent Document 1).
While use of this probe enables eddy current flaw detection for a weakly magnetic tube such as duplex phase stainless steel tube, it is not sensitive enough to detect small flaws in a ferromagnetic tube made of carbon steel or the like. Thus there is a demand for a method capable of accurately detecting small flaws in ferromagnetic tubes.
A magnetic body having relatively low saturation magnetic flux density such as 0.5 tesla or less may refer to weakly magnetic body herein and a magnetic body having relatively high saturation magnetic flux density such as 1.6 tesla or higher may refer to ferromagnetic body herein.